Technical requirements for heat treatment process of die-casting mold

1. Improved technology of traditional heat treatment process

The traditional heat treatment process of die-casting mold is quenching-tempering, and surface treatment technology has been developed later. Due to the variety of materials that can be used as die-casting molds, the same surface treatment technology and process applied to different materials will produce different effects. Schoff recently proposed a substrate pretreatment technology for mold substrate and surface treatment technology. On the basis of traditional technology, suitable processing techniques for different mold materials are proposed to improve mold performance and increase mold life.

Another development direction for the improvement of heat treatment technology is to combine traditional heat treatment technology with advanced surface treatment technology to increase the service life of die-casting molds. For example, the chemical heat treatment method carbonitriding, NQN (that is, carbonitriding-quenching-carbonitriding) combined with the conventional quenching and tempering process is combined to strengthen, not only to obtain higher surface hardness, but also effective hardening The layer depth increases, the hardness gradient distribution of the infiltrated layer is reasonable, the tempering stability and the corrosion resistance are improved, so that the die-casting mold obtains good core performance while greatly improving the surface quality and performance.

2. Surface modification technology

2.1. Surface thermal diffusion technology

This type includes carburizing, nitriding, boronizing, carbonitriding, sulfur carbonitriding and so on.

2.1.1, carburizing and carbonitriding

The carburizing process is used in cold, hot work and surface strengthening of plastic molds, which can improve the life of the mold. When carburizing, the main process methods include solid powder carburizing, gas carburizing, vacuum carburizing, ion carburizing, and carbonitriding formed by adding nitrogen to the carburizing atmosphere. Among them, vacuum carburizing and ion carburizing are technologies developed in the past 20 years. This technology has the characteristics of fast carburizing, uniform carburizing, smooth carbon concentration gradient and small workpiece deformation. Play an increasingly important role in surface treatment.

2.1.2 Nitriding and related low-temperature thermal expansion technology

This type includes nitriding, ion nitriding, carbonitriding, oxygen nitriding, sulphur nitriding and ternary sulphur carbon nitriding, oxygen, nitrogen and sulfur. These methods have simple processing technology, strong adaptability, low diffusion temperature (usually 480 ~ 600 ℃), small deformation of the workpiece, especially suitable for surface strengthening of precision molds, and high hardness of the nitride layer, good wear resistance, and better Good anti-sticking performance.

The nitriding process is a commonly used process for the surface treatment of die-casting molds, but when a thin and brittle white layer appears in the nitrided layer, it cannot resist the effect of alternating thermal stress, and it is easy to produce micro-cracks and reduce thermal fatigue resistance. Therefore, during the nitriding process, the process must be strictly controlled to avoid the generation of brittle layers. Recently, foreign countries have proposed the use of secondary and multiple nitriding processes.

The method of repeated nitriding can decompose the white bright nitride layer that is prone to microcracks during service, increase the thickness of the nitriding layer, and at the same time make the mold surface have a thick residual stress layer, so that the life of the mold can be significantly improved. In addition, there are methods such as salt bath carbonitriding and salt bath sulphur nitrocarburizing.

These processes are widely used in foreign countries and rarely seen in China. For example, in the TFI+ABI process, it is immersed in an alkaline oxidizing salt bath after nitrocarburizing in a salt bath. The surface of the workpiece is oxidized and appears black, and its wear resistance, corrosion resistance and heat resistance have been improved. The life of the aluminum alloy die-casting die treated by this method is increased by hundreds of hours. Another example is the oxynit process developed in France, where the nitrocarburizing process is followed by nitriding, which is more characteristic when applied to non-ferrous metal die-casting molds.

2.1.3 Boronizing

Due to the high hardness of the boronized layer (FeB: HV1800-2300, Fe2B: HV1300-1500), wear resistance and red hardness, as well as certain corrosion resistance and adhesion resistance, boronizing technology has achieved better results in the mold industry Application effect. However, due to the harsh working conditions of the die-casting mold, the boronizing process is rarely used in the surface treatment of the die-casting mold. However, in recent years, improved boronizing methods have emerged to solve the above problems and can be applied to the surface treatment of the die-casting mold. Such as multi-element, paint powder penetration and so on. The method of coating powder boronizing is to mix the boron compound and other penetrants and coat them on the surface of the die-casting mold. After the liquid evaporates, it is packed and sealed according to the general powder boronizing method, heated at 920°C and kept for 8 hours, followed by Air cooling. This method can obtain a dense and uniform infiltration layer, the hardness, wear resistance and bending strength of the surface infiltration layer of the mold are improved, and the service life of the mold is increased by more than 2 times on average.

2.1.4. Rare earth surface strengthening

In recent years, the method of adding rare earth elements in the surface strengthening of molds has been widely praised. This is because rare earth elements have multiple functions such as increasing the penetration rate, strengthening the surface and purifying the surface. It has a great impact on improving the surface structure of the mold, and the surface physical, chemical and mechanical properties. It can increase the penetration rate and strengthen On the surface, rare earth compounds are generated. At the same time, it can eliminate the harmful effects of trace impurities distributed on the grain boundary, and play the role of strengthening and stabilizing the grain boundary on the surface of the mold cavity.

In addition, rare earth elements interact with harmful elements in steel to generate high melting point compounds, which can inhibit the segregation of these harmful elements on the grain boundaries, thereby reducing deep brittleness. Adding rare earth elements to the surface strengthening treatment process of die-casting molds can significantly increase the thickness of the infiltration layer and increase the surface hardness of various infiltration methods. The cold and thermal fatigue properties are significantly improved, thereby greatly improving the life of the mold. At present, the treatment methods applied to the surface of the die-casting mold cavity include: rare earth carbonizing, rare earth carbonitriding, rare earth boronizing, rare earth boron aluminum, rare earth soft nitriding, rare earth sulfur nitrocarburizing, etc.

2.2. Laser surface treatment

Laser surface treatment is to use a laser beam for heating to quickly melt a thin layer of a certain depth on the surface of the workpiece. At the same time, vacuum evaporation, electroplating, ion implantation and other methods are used to coat alloy elements on the surface of the workpiece, and make it contact with the substrate under laser irradiation. The metal is fully fused, and after condensation, an alloy layer with a thickness of 10 to 1000 μm with special properties is obtained on the surface of the mold, and the cooling rate is equivalent to quenching. For example, the surface of H13 steel is processed by laser rapid melting process, the melting zone has high hardness and good thermal stability, high resistance to plastic deformation, and has obvious inhibition effect on the initiation and propagation of fatigue cracks.

3. Coating technology

As a kind of mold strengthening technology, coating technology is mainly used in plastic molds, glass molds, rubber molds, stamping molds and other mold surface treatments with relatively simple working environments. Die-casting molds need to withstand the harsh environment of alternating hot and cold stress, so coating technology is generally not used to strengthen the surface of die-casting molds. However, in recent years, it has been reported that the surface of die-casting molds has been strengthened by chemical composite plating to improve the anti-adhesion and mold release properties of the mold surface. In this method, (NiP)-PTFE composite plating is carried out after infiltrating polytetrafluoroethylene particles on an aluminum-based die-casting mold. Experiments have proved that this method is feasible in process and performance, and greatly reduces the friction coefficient of the mold surface.

4. Conclusion

Mold pressure processing is an important part of mechanical manufacturing, and the level, quality and life of the mold are closely related to the mold surface strengthening technology. With the advancement of science and technology, various mold surface treatment technologies have made great progress in recent years. appears in:

• ① Improvement of traditional heat treatment process and its combination with other new processes;
• ②Surface modification technology, including carburizing, low-temperature thermal diffusion (various nitriding, carbonitriding, ion nitriding, ternary co-infiltration, etc.), salt bath thermal diffusion, boronizing, rare earth surface strengthening, laser Surface treatment and electric spark deposition cermet, etc.;
• ③Coating technology and other aspects. However, for die-casting molds with extremely harsh working conditions, the existing new surface treatment processes cannot meet the ever-increasing requirements. It can be expected that more advanced technologies are also expected to be applied to the surface treatment of die-casting molds.

In view of the fact that surface treatment is one of the important means to improve the life of die-casting molds, to improve the overall level of die-casting mold production in China, surface treatment technology will play a pivotal role.

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